In vitro transcription systems are used to identify promoter signals and factors that control transcription in eucaryotic systems. These systems accurately initiate transcription from a wide variety of promoters. We find that for optimal transcription, proteins in the extracts bind to the DNA, and protect the DNA from micrococcal nuclease digestion. High jDNA concentrations inhibit trnascription, but precise transcription can be restored by the addition of the four histones H2A, H2B, H3 and H4.